As many users do, I use Thunderbird as my main Email Client and I'm pretty satisfied with it. However, receiving a huge amount of emails every day made me face the Filter section, which I find quite rusty for its potential. The current filters are surely ok, but it would be really better if the user could build complex filtering expressions, to easily group same-treatment emails under the same rules. It doesn't need to trample the current UX - it just needs the possibility to build nested boolean basic algebra, and that's it.
Furthermore, once the user is able to build an efficient "mail secretary" via these new set of rules, it would be critic to be able to export them into another Thunderbird instance using any open format file, like a JSON or anything else.